Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Currently, the highest yield strength linepipe in commercial use exhibits a yield strength of about 550 MPa (80 ksi). Higher strength linepipe steel is commercially available, e.g., up to about 690 MPa (100 ksi), but to our knowledge has not been commercially used for fabricating a pipeline. Furthermore, as is disclosed in U.S. Pat. Nos. 5,545,269, 5,545,270 and 5,531,842, of Koo and Luton, it has been found to be practical to produce superior strength steels having yield strengths of at least about 830 MPa (120 ksi) and tensile strengths of at least about 900 MPa (130 ksi), as precursors to linepipe. The strengths of the steels described by Koo and Luton in U.S. Pat. No. 5,545,269 are achieved by a balance between steel chemistry and processing techniques whereby a substantially uniform microstructure is produced that comprises primarily fine-grained, tempered martensite and bainite which are secondarily hardened by precipitates of .epsilon.-copper and certain carbides or nitrides or carbonitrides of vanadium, niobium and molybdenum.
In U.S. Pat. No. 5,545,269, Koo and Luton describe a method of making high strength steel wherein the steel is quenched from the finish hot rolling temperature to a temperature no higher than 400.degree. C. (752.degree. F.) at a rate of at least 20.degree. C./second (36.degree. F./second), preferably about 30.degree. C./second (54.degree. F./second), to produce primarily martensite and bainite microstructures. Furthermore, for the attainment of the desired microstructure and properties, the invention by Koo and Luton requires that the steel plate be subjected to a secondary hardening procedure by an additional processing step involving the tempering of the water cooled plate at a temperature no higher than the Ac.sub.1 , transformation point, i.e., the temperature at which austenite begins to form during heating, for a period of time sufficient to cause the precipitation of .epsilon.-copper and certain carbides or nitrides or carbonitrides of vanadium, niobium and molybdenum. The additional processing step of post-quench tempering adds significantly to the cost of the steel plate. It is desirable, therefore, to provide new processing methodologies for the steel that dispense with the tempering step while still attaining the desired mechanical properties. Furthermore, the tempering step, while necessary for the secondary hardening required to produce the desired microstructures and properties, also leads to a yield to tensile strength ratio of over 0.93. From the point of view of preferred pipeline design, it is desirable to keep the yield to tensile strength ratio lower than about 0.93, while maintaining high yield and tensile strengths.
There is a need for pipelines with higher strengths than are currently available to carry crude oil and natural gas over long distances. This need is driven by the necessity to (i) increase transport efficiency through the use of higher gas pressures and, (ii) decrease materials and laying costs by reducing the wall thickness and outside diameter. As a result the demand has increased for linepipe stronger than any that is currently available.
Consequently, an object of the current invention is to provide compositions of steel and processing alternatives for the production of low cost, low alloy, ultra-high strength steel plate, and linepipe fabricated therefrom, wherein the high strength properties are obtained without the need for a tempering step to produce secondary hardening. Furthermore, another object of the current invention is to provide high strength steel plate for linepipe that is suitable for pipeline design, wherein the yield to tensile strength ratio is less than about 0.93.
A problem relating to most high strength steels, i.e., steels having yield strengths greater than about 550 MPa (80 ksi), is the softening of the HAZ after welding. The HAZ may undergo local phase transformation or annealing during welding-induced thermal cycles, leading to a significant, i.e., up to about 15 percent or more, softening of the HAZ as compared to the base metal. While ultra-high strength steels have been produced with yield strengths of 830 MPa (120 ksi) or higher, these steels generally lack the toughness necessary for linepipe, and fail to meet the weldability requirements necessary for linepipe, because such materials have a relatively high Pcm (a well-known industry term used to express weldability), generally greater than about 0.35.
Consequently, another object of this invention is to produce low alloy, ultra-high strength steel plate, as a precursor for linepipe, having a yield strength at least about 690 MPa (100 ksi), a tensile strength of at least about 900 MPa (130 ksi), and sufficient toughness for applications at low temperatures, i.e., down to about -40.degree. C. (-40.degree. F.), while maintaining consistent product quality, and minimizing loss of strength in the HAZ during the welding-induced thermal cycle.
A further object of this invention is to provide an ultra-high strength steel with the toughness and weldability necessary for linepipe and having a Pcm of less than about 0.35. Although widely used in the context of weldability, both Pcm and Ceq (carbon equivalent), another well-known industry term used to express weldability, also reflect the hardenability of a steel, in that they provide guidance regarding the propensity of the steel to produce hard microstructures in the base metal. As used in this specification, Pcm is defined as: EQU Pcm=wt % C+wt % Si/30+(wt % Mn+wt % Cu+wt % Cr)/20+wt % Ni/60+wt % Mo/15+wt % V/10+5(wt % B);
and Ceq is defined as: EQU Ceq=wt % C+wt % Mn/6+(wt % Cr+wt % Mo+wt % V)/5+(wt % Cu+wt % Ni)/15.